The present invention relates generally to substrate handling and processing chambers, and, in particular, to heating a substrate support in such chambers.
Glass substrates are being used for applications such as active matrix television and computer displays, among others. Each glass substrate can form multiple display monitors each of which contains more than a million thin film transistors.
The glass substrates can have dimensions, for example, of 550 mm by 650 mm. The trend, however, is toward even larger substrate sizes, such as 650 mm by 830 mm and larger, to allow more displays to be formed on the substrate or to allow larger displays to be produced. The larger sizes place even greater demands on the capabilities of the processing systems.
The processing of large glass substrates often involves the performance of multiple sequential steps, including, for example, the performance of chemical vapor deposition (CVD) processes, physical vapor deposition (PVD) processes, or etch processes. Systems for processing glass substrates can include one or more process chambers for performing those processes.
Plasma-enhanced chemical vapor deposition (PECVD) is another process widely used in the processing of glass substrates for depositing layers of electronic materials on the substrates. In a PECVD process, a substrate is placed in a vacuum deposition chamber equipped with a pair of parallel plate electrodes. The substrate generally is mounted on a susceptor which also serves as the lower electrode. A flow of a reactant gas is provided in the deposition chamber through a gas inlet manifold which also serves as the upper electrode. A radio frequency (RF) voltage is applied between the two electrodes which generates an RF power sufficient to cause a plasma to be formed in the reactant gas. The plasma causes the reactant gas to decompose and deposit a layer of the desired material on the surface of the substrate body. Additional layers of other electronic materials can be deposited on the first layer by flowing another reactant gas into the chamber. Each reactant gas is subjected to a plasma which results in the deposition of a layer of the desired material.
Some problems associated with the processing of large glass substrates arise due to their unique thermal properties. For example, the relatively low thermal conductivity of glass makes it more difficult to heat or cool the substrate uniformly. In particular, thermal losses near the edges of any large-area, thin substrate tend to be greater than near the center of the substrate, resulting in a non-uniform temperature gradient across the substrate. The thermal properties of the glass substrate combined with its size, therefore, makes it more difficult to obtain uniform characteristics for the electronic components formed on different portions of the surface of a processed substrate. Moreover, heating or cooling the substrates quickly and uniformly is more difficult as a consequence of its poor thermal conductivity, thereby posing special challenges to achieving a high throughput.
To help obtain a more uniform temperature across large substrates, susceptors having multiple heating elements have been used. For example, some susceptors include inner and outer heating elements. The use of multiple heating elements, however, occasionally results in the susceptor becoming deformed as it is heated. One cause of the deformation is a temperature differential that can occur between the inner and outer heating elements. If the temperature differential, or gap, becomes too large, the thermal stresses in the susceptor can result in deformation of the susceptor and, in some instances, even breakage.